It is known in the art to use inks or compositions, comprising magnetic orientable magnetic or magnetizable pigment particles, particularly also optically variable magnetic or magnetizable pigment particles, for the production of security elements, e.g. in the field of security documents. Coatings or layers comprising oriented magnetic or magnetizable pigment particles are disclosed for example in U.S. Pat. Nos. 2,570,856; 3,676,273; 3,791,864; 5,630,877 and 5,364,689. Coatings or layers comprising oriented magnetic color-shifting pigment particles, resulting in particularly appealing optical effects, useful for the protection of security documents, have been disclosed in WO 2002/090002 A2 and WO 2005/002866 A1.
Security features, e.g. for security documents, can generally be classified into “covert” security features one the one hand, and “overt” security features on the other hand. The protection provided by covert security features relies on the concept that such features are difficult to detect, typically requiring specialized equipment and knowledge for detection, whereas “overt” security features rely on the concept of being easily detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile senses while still being difficult to produce and/or to copy. However, the effectiveness of overt security features depends to a great extent on their easy recognition as a security feature, because most users, and particularly those having no prior knowledge of the security features of a therewith secured document or item, will only then actually perform a security check based on said security feature if they have actual knowledge of their existence and nature.
Magnetic or magnetizable pigment particles in printing inks or coatings allow for the production of magnetically induced images, designs and/or patterns through the application of a corresponding magnetic field, causing a local orientation of the magnetic or magnetizable pigment particles in the coating, followed by hardening the latter. The result is a fixed magnetically induced image, design or pattern. Materials and technology for the orientation of magnetic or magnetizable pigment particles in coating compositions have been disclosed in U.S. Pat. Nos. 2,418,479; 2,570,856; 3,791,864, DE 2006848-A, U.S. Pat. Nos. 3,676,273, 5,364,689, 6,103,361, EP 0 406 667 B1; US 2002/0160194; US 2004/70062297; US 2004/0009308; EP 0 710 508 A1; WO 2002/09002 A2; WO 2003/000801 A2; WO 2005/002866 A1; WO 2006/061301 A1; these documents are incorporated herein by reference. In such a way, magnetically induced patterns which are highly resistant to counterfeit can be produced. The security element in question can only be produced by having access to both, the source of the magnetic or magnetizable pigment particles or the corresponding ink, and the particular technology employed to print said ink and to orient said pigment in the printed ink.
WO 2005/000585 A1 discloses printing machines comprising magnetic elements for orienting magnetic or magnetizable pigment particles. The disclosed magnetic elements are comprised in the impression cylinder. Alternatively, US 2005/000585 A1 discloses a stand-alone rotary magnetic orienting device, which can be used subsequent to a printing process, e.g. as an additional process station following in order to impose a particular orientation to magnetic or magnetizable pigment particles comprised in a freshly printed ink, prior to hardening (drying, curing) said ink.
EP 1 810 756 A2 discloses apparatus for orienting magnetic flakes, such as during a painting or printing process, to obtain an illusive optical effect. The disclosed apparatus comprises a rotatable roller comprising a non-magnetic cylindrical body having cavities formed therein and permanent magnets positioned in said cavities for forming magnetized portions of the roller, the one or more permanent magnets shaped for creating the magnetic field of the pre-determined configuration. Alternatively, EP 1 810 756 A2 discloses a cylindrical body encased by a flexible sheet of a magnetic material which is selectively magnetized for providing magnetized portions of the roller.
WO 2010/066838 A1 discloses a device for producing indicia comprising magnetically oriented magnetic or magnetizable particles in an ink or coating composition on a sheet of substrate material. The disclosed device comprises a flat-bed screen-printing and a printing platen for receiving said sheet, said printing platen having an upper surface facing the printing screen and a first direction along its upper surface along which said sheet is unloadable, and a magnetic orienting unit comprising multiple magnet assemblies. The magnetic orienting unit is disposed below the upper surface of the printing platen and all of said magnet assemblies are concomitantly movable from a first position away from the upper surface of the printing platen to a second position close to the upper surface of the printing platen.
A need remains for printing devices for high-speed productions of magnetically induced optical effect layers, said devices providing an increased contact time between the magnetic elements and the not yet hardened coating composition comprising magnetic or magnetizable pigment particles without the dimensional constraints of conventional cylindrical bodies having cavities comprising the magnetic elements and while allowing freedom in terms of the choice of the printing process and coating compositions comprising magnetic or magnetizable pigment particles.